Updated: Cycles show missing Texture (magenta)... until i reconnect the materialnodes
Found a solution! Converted the 16-bit png-files into 8-bit png-files. With these the problem doesnt seem to appear. But i'm still curious... perhaps someone has an idea why that happens with the 16bit Textures.

I'll have a look at the bugtracker... to see if it's been reported. If not i'll post that in there. (Thanks Matt!)